1. Field of the Invention
This invention relates to binders for porous electrodes used as carbon electrode of electric double layer capacitors, electrolytic capacitors and lithium ion secondary batteries and novel porous electrodes using the same binders.
2. Prior Art
Carbon electrodes using active carbon find extensive applications as anode and cathode electrodes of electric double layer capacitors, cathode electrodes of lithium ion secondary batteries, etc. The carbon electrode is manufactured by coating or depositing a composition, which is obtained by using a dispersion of polytetrafluoroethylene (PTFE) as binder and incorporating active carbon, carbon black or conductive polymer, on a base such as an aluminum foil and drying the coating.
The electrostatic capacitance of carbon electrodes or the like for electric double layer capacitors and lithium ion secondary batteries depend on the contact area (i.e., effective contact area) between conductive fine particles and electrolyte power unit projection area of electrode foil. Specifically, the electrostatic capacitance is closely related to such parameters as the specific area of conductive fine particles themselves and the deposition density of conductive fine particles per unit projection area of electrode foil.
However, an upper limit is imposed on the specific area of conductive fine particles, and revolutional increase of the electrostatic capacitance by movement of the specific area can be hardly expected. It is thus thought to be important how efficiently conductive fine particles are to be deposited on the electrode foil. PTFE which is used as binder in the carbon electrode has lower adhesion, and it is thus inferior in such connection as the conductive fine particle deposition property and the close contact between the conductive fine particles and the electrode foil. Also, since the binder has lower adhesion, the conductive fine particles are readily separated from the electrode foil, and the binder is thus inferior in connection with the resistance against vibrations. The above drawbacks may be improved by adding a great quantity of binder. However, by increasing the quantity of binder added the resistance of the electrode is correspondingly increased. At any rate, it is impossible to obtain a sufficiently practical electrode.
The inventor accordingly proposed in Japanese Patent Laid-Open No. 11-135379 that a porous electrode material having excellent electrostatic capacitance is obtainable by dispersing conductive film particles in porous rubber.
Thus electrode material has excellent electrostatic capacitance. However, pressing the electrode for improving the electrostatic capacitance per unit volume, gives rise to a problem that the conductive fine particles and the porous rubber in the electrode material are mashed to result in reduction of the electrostatic capacity. In addition, there is some demand for improving the material in the aspect of the close contact between the porous electrode and a conductive base (i.e., collector electrode).
The invention has an object of providing an electric material having excellent electrostatic capacitance and low resistance by improving the conductive fine particle deposition factor and also improving the close contact between conductive fine particles and conductive base.
Another object of the invention is to provide an electrode material, which is excellent in adhesion, solvent resistance, vibration resistance and other properties.